Current regulator for carpet seaming apparatus

ABSTRACT

Apparatus for controlling the current flow through metal foil heating elements in carpet joining tape comprises an oscillator which produces a high frequency output to drive a power amplifier so that the output voltage to be applied to the heating element is at a frequency proportional to or the same as the oscilltor frequency. The output current (Rms) value is controlled by controlling the pulse with using a modulator which either increases or reduces the output pulse width in accordance with a comparison of the sensed output current against a reference derived from a control circuit. The reference voltage is dependent on the operating mode of the apparatus selected by switches or switch and counter.

BACKGROUND OF THE INVENTION

This invention relates to improvements in carpet seaming apparatus andrelates particularly to improved apparatus for use with a carpet seamingtape or the like to produce a seam between adjacent edge portions of acarpet or other material.

In my Australian Pat. No. 529,440 and Patent Application No. 10491/83, Ihave described apparatus which includes means for passing a currentthrough a strip of metal foil which is associated with or incorporatedin a carpet joining tape, the apparatus having means to control thecurrent flow through the metal foil so that heat generated thereby willact to soften an adhesive on the carpet joining tape, the apparatusfurther including means to hold the current flow at a selectedpredetermined value to maintain the adhesive in a softened condition andmeans selectively operated to increase the current to fully melt theadhesive.

The particular form of apparatus previously described utilized a powertransformer the output of which was controlled by a Triac which, inturn, was controlled by a trigger module which sensed both the outputvoltage and current. A rheostat was used to vary the output voltage andcurrent as desired.

While this apparatus functions well in most circumstances, it has beenfound that the weight of the apparatus is relatively high thus making itdifficult to move from place to place. Further, the components of theapparatus must be completely redesigned for use on the various overseasmain power systems.

SUMMARY OF THE INVENTION

It is desirable to provide improved apparatus for use in formingedge-to-edge seams in carpets or other sheet material which avoids atleast some of the above disadvantages.

It is also desirable to provide an improved carpet seaming apparatuswhich is economical to manufacture and is relatively safe to use.

According to one aspect of the invention there is provided apparatus foruse in joining abutting edges of carpet or other material using aheating element in conjunction with a joining tape having heatsoftenable adhesive thereon, said apparatus comprising an oscillatormeans having an output frequency in the range of 100 to 50,000 hertz,power output means driven by the oscillator to produce an output voltagehaving a frequency which is a function of the oscillator meansfrequency, pulse width modulator means to vary the pulse width of theoutput voltage to thereby vary the output current which is to be appliedto the heating element, sensing means to sense the output current,control means to adjust the pulse width modulator means in accordancewith sensed output current to thereby maintain a predetermined selectedoutput current, and switch means operable to select the desired outputcurrent.

In a preferred embodiment, the power output means comprises a poweramplifier which is driven by the oscillator means. The pulse widthmodulator means controls the width of the output pulses of theoscillator means which thereby controls the Rms value of the outputcurrent.

A differential amplifier receives a reference voltage from a referencevoltage source and a signal voltage derived from the output currentsensing means, which may comprise a current transformer and Rms-to-DCconvertor. Any differences between the reference and signal voltagesresults in the differential amplifier providing an output signal to thepulse width modulator means which either increases or decreases theoutput current pulse width to reduce the voltage differences.

The switch means is operable to change the selected reference voltage tothereby change the selected desired output current.

According to another aspect of this invention there is providedapparatus for use in joining abutting edges of carpet or other materialusing a heating element in conjunction with a joining tape having heatsoftenable adhesive thereon, said apparatus comprising an oscillatorhaving an output frequency of between 100 and 50,000 hertz, a pulsewidth modulator to adjust the pulse width of the oscillator output, adrive amplifier driven by the output of the pulse width modulator andproducing an output voltage of the oscillator frequency to be applied tothe heating element to cause a heating current to pass therethrough,means to sense the output current, means to adjust the pulse widthmodulator output in accordance with sensed output current to therebycontrol the output, means selectively operable to maintain apredetermined current output and further means to selectively increasethe current output.

In a preferred form of this aspect of the invention, the output from thedrive amplifier is applied to an output transformer, the output from thesecondary of which is adapted to be connected to the heating element.

In an alternative arrangement, the output transformer may be replaced bydiodes thereby producing a pulsed d.c. output current.

Preferably, the oscillator produces a square wave having a frequency ofbetween 400 and 3,000 hertz. This output, after modulation by the pulsewidth modulator, drives the drive amplifier to produce an output currentof the oscillator frequency.

When the high frequency output is applied to a heating element, such asa strip of foil incorporated in a carpet seaming tape, the element isheated to thereby soften adhesive on the tape. It is believed that thehigh frequency of the current passing through the element produces"skin-effect" heating which enables the adhesive to be heated relativelyrapidly. When the element has heated to a temperature sufficient tosoften the adhesive, a switch is operated whereby the output current isreduced and is maintained at a level sufficient to maintain the elementat the adhesive softening temperature for a period of time sufficient toenable the abutting edges of the carpet to be properly aligned on thetape in the manner described in my aforesaid Australian Patent. Theadhesive, at this time, is in a tacky condition which enables the carpetedge portions to be properly adjusted. On completion of adjustment, aswitch is actuated whereby the current flow is increased to therebyincrease the temperature of the element and thus fully melt the adhesivewhereby, on rolling, the adhesive flows into the carpet backing which,it has been found, produces a secure, water-proof and shampoo-proofseam. A timer may be used to switch off the current flow after apredetermined time to prevent overheating of the seaming tape element.

The means to sense the output current and to enable the current to bemaintained at a level sufficient to maintain the temperature of theheating element may comprise a current transformer in the output circuitwhich provides a signal to the pulse with modulator. The signal is usedto adjust the pulse width of the drive and wave form into the driveamplifier. Therefore, if for example, as a result of cooling of theheating element, the output current is sensed to change, due to thechange in foil resistance, the feedback circuit senses that change andmakes appropriate adjustment to the output current to maintain thedesired heating element temperature.

The apparatus of the present invention is relatively light in weight ascompared to previous apparatus due largely to the absence of a powertransformer designed to operate at normal supply frequencies. Theapparatus of the present invention also has a higher duty cycle ascompared to previous apparatus, the full current output being able to bemaintained indefinitely. Further, the present invention is relativelysimply adapted for use with the various power supplies of overseascountries.

In order that the invention is more readily understood, two embodimentsthereof will be described with reference to the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of one embodiment of the invention, and

FIG. 2 is a block diagram of a second embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the input 12 is adapted to be connected to aconventional power outlet which, in Australia, is of 240 volts at 50 Htzwith a rated output of 10 amps. The mains supply is transformed by therelatively small transformer 13 to provide 20 volts for operation of thecontrol circuitry. The main supply is also applied to an A.C./D.C.converter 15, the D.C. output of which passes to a drive amplifier 18.

The control circuitry comprises an operating control circuit 14, anoscillator 16 producing an output voltage having a frequency of about1000 Hertz, and a pulse width modulator 17. The output from the pulsewidth modulator 17 is used to drive the drive amplifier 18 to produce anoutput voltage of the oscillator frequency which passes through arelatively small output transformer 19 to output terminals 21. A currenttransformer 20 senses output current, this information being fed back tothe pulse width modulator 17.

In use, the input 12 is connected to mains power and the outputterminals 21 are connected to a strip of metal foil in a carpet seamingtape. The mains power is converted by the converters 15 to a D.C.current which is fed to the drive amplifier 18. The oscillator 16produces a square wave output and the pulse width of this is controlledby the pulse width modulator 17 which drives the drive amplifier 18.

In a first operating mode, such as, for example, when the apparatusoutput is first connected to the metal foil and actuated, the controlcircuit 14 provides a signal to the pulse width modulator whereby theoscillator output pulses are modulated to a predetermined maximum widthso that the output current from the drive amplifier 18 is apredetermined maximum.

The current transformer 20 senses the output current flowing and causesthe pulse width modulator 17 to increase or decrease the pulse width inorder to maintain the predetermined value in accordance with the controlcircuit signal.

When the foil temperature rises sufficiently to just soften the adhesiveon the carpet joining tape, the push button 22 is actuated and thecontrol circuit 14 switches to a "HOLD" mode. In this mode, the signalprovided by the control circuit 14 reduces the pulse width of theoscillator output pulses so that the output current is accordinglyreduced to a predetermined level, which is sufficient to maintain thefoil temperature. During this time, the edges of the carpet being joinedcan be adjusted. On completion of adjustments, the push button 23 isactuated to increase the current flowing through the foil to therebyfully melt the adhesive. When the adhesive has melted, the apparatus iselectrically disconnected from the foil so that no further current flowstherethrough.

During the "hold" mode, any loss or increase of temperature of the foilis reflected in a change in the current flowing, due to a change ofresistance. This change is compensated by the current transformer 17 andpulse width modulator.

If desired, a timer may be incorporated in the control circuit to switchoff the apparatus a predetermined time after the button 23 has beenactuated. Further, ambient temperature compensation may be incorporatedinto the control circuit to vary the output during the "hold" phase inaccordance with variations in sensed ambient temperature. Thus, if theapparatus is being used in direct sunlight or on outdoor carpet or othermaterial, the output during the "hold" phase is reduced as heat lossesfrom the heating element i.e. the foil, will also be reduced as comparedto such losses when the apparatus is used in a relatively cold ambienttemperature.

In the past, prior apparatus sometimes incorporated a timer whichcontrolled the heating cycle and turned off the current before theelement became too hot. However, such apparatus did not operatecorrectly in cold ambient temperatures or hot temperatures as thecurrent was switched off either before the adhesive became fully meltedor after the carpet overheated and burnt or deteriorated. The presentinvention can incorporate the compensating means in conjunction with thetimer to lengthen or shorten the "heat" cycle in accordance with theambient temperature so that the current is cut off at the correctadhesive condition. An audible and/or visual alarm can be included toindicate to the operator that the seam is ready to roll.

In a modified arrangement, a further control switch may be provided inthe control circuit to enable manual selection of a lower output in the"hold" phase if the operator determines that the normal output isresulting in too great a temperature of the foil. This situation mayoccur where the length of the seam is relatively short and theresistance of the heating element is accordingly lower.

Referring to FIG. 2 of the drawings, a modified form of the invention isillustrated schematically. In this embodiment, the input voltage appliedto input terminals 26 is rectified and filtered in the rectifier 27 toprovide a D.C. output voltage of approximately 300 volts. This voltageis applied to the inverter 28 which incorporates an oscillator circuitand a power amplifier stage.

The output of the inverter 28 is approximately 300 V (Rms) of afrequency the same as or proportional to the oscillator frequency.Preferably, the output voltage frequency is about 4000 Hertz, althoughthis can vary in accordance with other design requirements within therange of 400 Hertz to 50 K Hertz, depending on the proposed applicationof the apparatus.

A relatively small output transformer 29 (compared to a 50 Hertztransformer) reduces the output voltage to about 60 V at the outputterminals 30.

The pulse width of the output current, and therefore the Rms value ofthe output current, is controlled by the pulse width modulator 32 toincrease or decrease the Rms output current in accordance with thecurrent sensed by the current transformer 31 compared with apredetermined reference. An Rms to D.C. converter 33 takes the currenttransformer 31 output to produce a D.C. signal proportional to the Rmsvalue. This signal is fed to the inverting input 34 of the differentialamplifier 36. A reference voltage is applied to the non-inverting input37 of the differential amplifier 36 so that the output therefrom is afunction of the difference between the inputs 34 and 37. This differencesignal applied to the pulse width modulator 32 causes an increase ordecrease of output current pulse width necessary to reduce thedifference signal to zero.

The reference voltage applied to the differential amplifier input 37 isderived from the reference voltage source 38 which receives inputs fromcounter 39. A selector switch 41 initiates the counter 39 tosequentially switch between four operating modes, including an OFF mode.In the OFF mode, the reference voltage source 38 produces a zero outputsignal to the differential amplifier and, hence, the output current iszero.

Actuation of the selector switch 41 steps the counter 39 to the PRE-HEATmode whereby the reference voltage applied to the differential amplifier36 causes heating current to flow through the metal foil to which theoutput terminals 30 are connected. The maximum Rms value of outputcurrent is controlled by the monitoring current transformer 31, the Rmsto D.C. converter 33 and differential amplifier 36.

When the heating element (metal foil) has reached the desiredtemperature, i.e. when the adhesive on the carpet joining tape becomestacky, the selector switch 41 steps the counter to the PAUSE mode. Inthis mode, the reference voltage is reduced to a predetermined levelwhich adjusts the pulse width modulator to reduce the output current toa level which is predetermined to be sufficient to maintain the adhesivein the tacky condition. In this embodiment, the reference voltage and,hence the output current is reduced to about 70% of that in the PRE-HEATmode, although this can be varied in any particular form of apparatusdepending on its application. Manual variation may, if desired, beincorporated in the reference voltage source.

After appropriate adjustment of the carpet seam, a further actuation ofthe selector switch 41 steps the counter 39 to the WELD mode in whichthe output current is increased to fully melt the adhesive and form theseam, as described in my earlir patent and patent application aforesaid.Either an inbuilt timer or actuation of the selector switch 41 steps thecounter back to the OFF setting.

A mode display 43 provides a visual indication of the operating modestatus of the apparatus. An audible alarm may also be incorporated intothe counter 39 and associated with the timer to give an audible alarmafter predetermined time periods after initiation of the PRE-HEAT andWELD modes so that overheating can be avoided.

The control circuitry comprising the counter 39, reference voltagesource 38, pulse width modulator 32 and Rms to D.C. converter 33 derivespower from the transformer 44.

In this embodiment the high frequency output current derived from theinverter 28 enables rapid and controlled heating of the metal foilheating element, whether that element is incorporated in a carpetseaming tape or comprises the seaming/unseaming tape described in myInternational Patent Application No. PCT/AU82/00222.

Ambient temperature compensation is incorporated into the referencevoltage source 38 to take account of operating conditions, as decribedin the previous embodiment.

The apparatus of the invention may be modified in accordance with thedesign parameters referred to above and in accordance with latestdevelopments in the art of electrical control circuitry. Suchmodifications may include the provision of filters to remove mainsspikes, the use of integrated circuitry in the control circuitprogrammed to vary the oscillator output, the pulse width modulatorand/or the output power amplifier in accordance with sensed, measured ormanually set input data to control the output to that which is optimumfor each operation. Thus, the control circuits may include an impedencemeasuring function which, when the input of the apparatus is connectedto the heating element, measures the impedence thereof and controls theheating operation in accordance with that measured impedence. Otherpreprogrammed control functions which may be incorporated into theapparatus of the invention include preprogramming a microprocessor totake account of known foil heating parameters to automatically controlthe heating currents and time, given the seam length.

I claim:
 1. Apparatus for use in joining abutting edges of carpet orother material using a resistive heating element in conjunction with ajoining tape, the joining tape having a heat softenable adhesivethereon, said apparatus comprising power input means to be connected toan A.C. mains power source, an oscillator means having an outputfrequency in the range of 100 to 50,000 Hertz, power output means drivenby the oscillator to produce an A.C. or pulsed D.C. output voltagehaving a frequency which is a function of the oscillator meansfrequency, said power output means including an amplifier meansreceiving a control signal from a pulse width modulator to vary thepulse width of the oscillator means output which varies both the pulsewidth of the output voltage and the Rms value of output current which isapplied to the heating element, current sensing means to sense the Rmsvalue of output current, control means to adjust the pulse widthmodulator means in accordance with sensed output current to mainstain apredetermined selected output current, and switch means operable toselect the desired output current, the switch means having a firstoperational mode in which the output voltage applied to the heatingelement has a frequency and a pulse width to cause the Rms value of theoutput current to be at a first predetermined value at which heating ofthe heating element occurs and a second operational mode in which thepulse width is reduced to reduce the Rms value of the output current toa value which maintains the heating element at a desired temperature. 2.Apparatus according to claim 1 wherein the output voltage frequency isbetween 400 and 5000 Hertz.
 3. Apparatus according to claim 1 whereinthe output frequency is about 3000 Hertz.
 4. Apparatus according to anyone of the preceding claims wherein the pulse width modulator meansvaries the pulse width of the oscillator output to thereby effect acorresponding variation of the output current.
 5. The apparatus for usein joining abutting edges of carpet or other material as set forth inclaim 1, wherein said current sensing means comprises a currenttransformer in the output circuit.
 6. The apparatus for use in joiningabutting edges of carpet or other material as set forth in claim 1,wherein said control means comprises a reference voltage source, switchmeans to select a desired reference voltage, and comparator means tocompare the sensed output current with the selected reference voltage.